1. Field of the Invention
The present invention relates to a thermoplastic composite material. Specifically the invention relates to a thermoplastic composite material which may be manufactured in sheet or panel form and with the composite material later thermoformed into a complex shape having complex contours. In particular, the complex shape may be used for various industrial or medical purposes and one such specific example is an orthotic insert for use within foot gear.
2. Description of Prior Art
It is often desirable to form a sheet of plastic material into a complex shape. One method of accomplishing this is to provide for a mold having the desired complex shape and to thermoform the plastic sheet into the complex shape. The thermoforming is typically accomplished through the use of heat and pressure, such as vacuum forming, to conform the plastic sheet to the complex shape. In this type of structure, the plastic sheet would be a thermoplastic material which may be manufactured and sold in sheet form and with a subsequent forming into the complex shape as described above.
As an example, orthotic inserts are currently made using a sheet of acrylic material. The sheet of acrylic material may be cut into a desirable shape and then molded under heat and vacuum to conform to the shape of a casting made in representation of the bottom of a foot. Other thermoplastic materials may be used in substitution to acrylics such as polypropylenes and polyethylenes. The difficulty with all of these materials for use in structures such as orthotics is that, in order to provide the proper level of rigidity necessary to properly serve as an orthotic, these prior art materials are generally quite thick and as an example may vary between 120 to 200 thousandths of an inch. Unfortunately, such thick materials are heavy and are generally too thick to slip into the foot gear already owned. This requires the purchase of larger foot gear to accommodate these prior art orthotic inserts. In addition, the prior art materials are not only thick, but are not as durable and resistant to breaking as would be desirable.
As an alternative to the prior art thermoplastic materials, thermoset laminate constructions have been used to provide for complex shapes such as orthotic inserts. As an example, reference is made to prior U.S. Pat. No. 4,439,934 issued Apr. 3, 1984, for an orthotic insert. In this patent an insert is formed by a cumbersome technique of laying upon a positive cast a series of layers of various material. These layers may include layers of cloth impregnated with thermoset resin and layers of graphite and with these individual layers thermoset to the desired shape under heat. If the cast is not perfectly accurate, then the insert cannot be adjusted since it has been thermoset (cured) to a specific structural form.
The use of the thermoset process, therefore, means that the orthotic insert must be manufactured to the exact shape the first time since the thermoset process does not allow for any post forming at a later time in the field. This limitation has, therefore, greatly restricted the use of such thermoset laminate constructions as shown in the above referenced patent. It is much more desirable for the person prescribing the orthotic insert to be able to post form the insert to make adjustments to the orthotic insert. This is acomplished through the use of localized heating to make changes in the complex shape of the insert without in turn destroying the structure or integrity of the insert. For these reasons, thermoplastic orthotic inserts have received far greater acceptance in the orthotic field even with the various limitations described above.
It can be seen that a proper combination of features is not available in the prior art. In general the prior art acrylic material is relatively easy to fabricate and is post formable, but this material is typically quite thick, such as 120 to 180 thousandths of an inch and is both brittle and heavy. Other prior art materials such as polypropylene are also quite thick, such as 160 to 200 thousandths of an inch, but this material is more difficult to fabricate than acrylic and is more difficult to post form. The polypropylene material is heavy, but it is not brittle. The thermoset laminate described above can be quite thin, such as 65 to 80 thousandths of an inch. However, the fabrication of the thermoset sandwich is difficult and generally requires special processing and once formed is not post formable. The formed thermoset material is not brittle, but even though the material is quite thin it is heavy.
Because of the above described problems with the thermoset laminate construction, complex shapes, such as orthotic inserts, are generally formed using thermoplastic materials, such as acrylic and polypropylene. In particular the inserts are made by first making a plaster mold from the patient's foot. This plaster mold forms a negative image of the bottom of the foot. A positive mold is then made from the negative plaster casting and thermoplastic sheet material is thermoformed using heat and vacuum to conform to the positive mold to make the appropriate insert. As indicated above, thermoplastic material has the great advantage in that it is post formable so that the orthotic insert may be heat adjusted at a later time to correct any minor misfit. This is important since it eliminates the necessity of making a new insert as would be necessary with a thermoset laminate constuction.